Interchangeable means able to be swapped or exchanged without making any real difference. When two things are interchangeable, you can use one in place of the other and everything works just as well.

In everyday life, interchangeable parts make modern manufacturing possible. Before the 1800s, each musket or clock was handmade with custom parts. If a piece broke, a craftsperson had to create a new one specially fitted to that particular item. Then inventors developed interchangeable parts: identical pieces that could replace each other perfectly. Now when your bike chain breaks, any chain of the right size works, because manufacturers make them interchangeable.

The word also describes things that are so similar they could substitute for each other. If your teacher says two words are interchangeable, you can use either one without changing the meaning. If someone complains that their siblings get treated as interchangeable, they feel like their parents don't recognize what makes each of them unique.